Methods for determining meniscal size and shape and for devising treatment

ABSTRACT

The present invention relates to methods for determining meniscal size and shape for use in designing therapies for the treatment of various joint diseases. The invention uses an image of a joint that is processed for analysis. Analysis can include, for example, generating a thickness map, a cartilage curve, or a point cloud. This information is used to determine the extent of the cartilage defect or damage and to design an appropriate therapy, including, for example, an implant. Adjustments to the designed therapy are made to account for the materials used.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/853,599 filed on Aug. 10, 2010 which in turn is a continuation ofU.S. patent application Ser. No. 10/704,325 filed on Nov. 7, 2003, whichin turn claims priority to U.S. Provisional Patent Application60/424,964 filed on Nov. 7, 2002.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

Certain aspects of the invention described below were made with UnitedStates Government support under Advanced Technology Program 70NANBOH3016awarded by the National Institute of Standards and Technology (NIST).The United States Government may have rights in certain of theseinventions.

FIELD OF THE INVENTION

The present invention relates to methods for determining meniscal sizeand shape for use in designing therapies for the treatment of variousjoint diseases. This method is then used to design an implant orarticular repair system for use in a joint.

BACKGROUND OF THE INVENTION

There are various types of cartilage, e.g., hyaline cartilage andfibrocartilage. Hyaline cartilage is found at the articular surfaces ofbones, e.g., in the joints, and is responsible for providing the smoothgliding motion characteristic of moveable joints. Articular cartilage isfirmly attached to the underlying bones and measures typically less than5 mm in thickness in human joints, with considerable variation dependingon the joint and more particularly the site within the joint. Inaddition, articular cartilage is aneural, avascular, and alymphatic

Adult cartilage has a limited ability of repair; thus, damage tocartilage produced by disease, such as rheumatoid arthritis and/orosteoarthritis, or trauma can lead to serious physical deformity anddebilitation. Furthermore, as human articular cartilage ages, itstensile properties change. Thus, the tensile stiffness and strength ofadult cartilage decreases markedly over time as a result of the agingprocess.

For example, the superficial zone of the knee articular cartilageexhibits an increase in tensile strength up to the third decade of life,after which it decreases markedly with age as detectable damage to typeII collagen occurs at the articular surface. The deep zone cartilagealso exhibits a progressive decrease in tensile strength with increasingage, although collagen content does not appear to decrease. Theseobservations indicate that there are changes in mechanical and, hence,structural organization of cartilage with aging that, if sufficientlydeveloped, can predispose cartilage to traumatic damage.

Once damage occurs, joint repair can be addressed through a number ofapproaches. The use of matrices, tissue scaffolds or other carriersimplanted with cells (e.g., chondrocytes, chondrocyte progenitors,stromal cells, mesenchymal stem cells, etc.) has been described as apotential treatment for cartilage and meniscal repair or replacement.See, also, International Publications WO 99/51719 to Fofonoff, publishedOct. 14, 1999; WO01/91672 to Simon et al., published Dec. 6, 2001; andWO01/17463 to Mannsmann, published Mar. 15, 2001; U.S. Pat. No.6,283,980 B1 to Vibe-Hansen et al., issued Sep. 4, 2001, U.S. Pat. No.5,842,477 to Naughton issued Dec. 1, 1998, U.S. Pat. No. 5,769,899 toSchwartz et al. issued Jun. 23, 1998, U.S. Pat. No. 4,609,551 to Caplanet al. issued Sep. 2, 1986, U.S. Pat. No. 5,041,138 to Vacanti et al.issued Aug. 29, 1991, U.S. Pat. No. 5,197,985 to Caplan et al. issuedMar. 30, 1993, U.S. Pat. No. 5,226,914 to Caplan et al. issued Jul. 13,1993, U.S. Pat. No. 6,328,765 to Hardwick et al. issued Dec. 11, 2001,U.S. Pat. No. 6,281,195 to Rueger et al. issued Aug. 28, 2001, and U.S.Pat. No. 4,846,835 to Grande issued Jul. 11, 1989. However, clinicaloutcomes with biologic replacement materials such as allograft andautograft systems and tissue scaffolds have been uncertain since most ofthese materials cannot achieve a morphologic arrangement or structuresimilar to or identical to that of normal, disease-free human tissue itis intended to replace. Moreover, the mechanical durability of thesebiologic replacement materials remains uncertain.

Usually, severe damage or loss of cartilage is treated by replacement ofthe joint with a prosthetic material, for example, silicone, e.g. forcosmetic repairs, or suitable metal alloys. See, e.g., U.S. Pat. No.6,443,991 B1 to Running issued Sep. 3, 2002, U.S. Pat. No. 6,387,131 B1to Miehlke et al. issued May 14, 2002; U.S. Pat. No. 6,383,228 toSchmotzer issued May 7, 2002; U.S. Pat. No. 6,344,059 B1 to Krakovits etal. issued Feb. 5, 1002; U.S. Pat. No. 6,203,576 to Afriat et al. issuedMar. 20, 2001; U.S. Pat. No. 6,126,690 to Ateshian et al. issued Oct. 3,2000; U.S. Pat. No. 6,013,103 to Kaufman et al. issued Jan. 11, 2000.Implantation of these prosthetic devices is usually associated with lossof underlying tissue and bone without recovery of the full functionallowed by the original cartilage and, with some devices, seriouslong-term complications associated with the loss of significant amountsof tissue and bone can include infection, osteolysis and also looseningof the implant.

As can be appreciated, joint arthroplasties are highly invasive andrequire surgical resection of the entire, or a majority of the,articular surface of one or more bones involved in the repair. Typicallywith these procedures, the marrow space is fairly extensively reamed inorder to fit the stem of the prosthesis within the bone. Reaming resultsin a loss of the patient's bone stock and over time subsequentosteolysis will frequently lead to loosening of the prosthesis. Further,the area where the implant and the bone mate degrades over timerequiring the prosthesis to eventually be replaced. Since the patient'sbone stock is limited, the number of possible replacement surgeries isalso limited for joint arthroplasty. In short, over the course of 15 to20 years, and in some cases even shorter time periods, the patient canrun out of therapeutic options ultimately resulting in a painful,non-functional joint.

U.S. Pat. No. 6,206,927 to Fell, et al., issued Mar. 27, 2001, and U.S.Pat. No. 6,558,421 to Fell, et al., issued May 6, 2003, disclose asurgically implantable knee prosthesis that does not require boneresection. This prosthesis is described as substantially elliptical inshape with one or more straight edges. Accordingly, these devices arenot designed to substantially conform to the actual shape (contour) ofthe remaining cartilage in vivo and/or the underlying bone. Thus,integration of the implant can be extremely difficult due to differencesin thickness and curvature between the patient's surrounding cartilageand/or the underlying subchondral bone and the prosthesis.

Interpositional knee devices that are not attached to both the tibia andfemur have been described. For example, Platt et al. (1969) “MouldArthroplasty of the Knee,” Journal of Bone and Joint Surgery 51B(1):76-87, describes a hemi-arthroplasty with a convex undersurfacethat was not rigidly attached to the tibia.

U.S. Pat. No. 4,502,161 to Wall issued Mar. 5, 1985, describes aprosthetic meniscus constructed from materials such as silicone rubberor Teflon with reinforcing materials of stainless steel or nylonstrands. U.S. Pat. No. 4,085,466 to Goodfellow et al. issued Mar. 25,1978, describes a meniscal component made from plastic materials.Reconstruction of meniscal lesions has also been attempted withcarbon-fiber-polyurethane-poly (L-lactide). Leeslag, et al., Biologicaland Biomechanical Performance of Biomaterials (Christel et al., eds.)Elsevier Science Publishers B.V., Amsterdam. 1986. pp. 347-352.Reconstruction of meniscal lesions is also possible with bioresorbablematerials and tissue scaffolds.

However, currently available devices do not always provide idealalignment with the articular surfaces and the resultant joint congruity.Poor alignment and poor joint congruity can, for example, lead toinstability of the joint. In the knee joint, instability typicallymanifests as a lateral instability of the joint.

Thus, there remains a need for methods that recreate natural or nearnatural relationships between two articular surfaces of the joint (suchas the femoral condyle and the tibial plateau).

SUMMARY OF THE INVENTION

In one aspect, when the meniscus is present in the subject, theinvention includes measuring the dimensions and/or shape parameters ofthe meniscus. Such dimensions and parameters include, for example, butare not limited to, the maximum anterior-posterior distance of themeniscus, the maximum medial-lateral distance of the meniscus, the sizeor area of the meniscal attachment(s), the maximum length of theanterior horn, the maximum and minimum height of the anterior horn, themaximum and minimum height of the body, the maximum and minimum heightof the posterior horn, the maximum height and minimum height of themeniscus, the maximum and minimum width of the anterior horn, themaximum and minimum width of the body, the maximum and minimum width ofthe posterior horn, meniscal radii and angles at various locations.These measurements can then be used to design therapies for thetreatment of joint diseases. These treatments can include, for example,meniscal repair systems, cartilage repair systems, articular repairsystems and arthroplasty systems and they can consist of, for example,biologic materials, tissue scaffolds, plastic, metal or metal alloys, orcombinations thereof. Therapies can be custom-made, typically utilizingat least one or more of these measurements. Alternatively, a pre-made,“off-the-shelf” component closely matching at least one or more of thesemeasurements can be selected.

In another aspect, the invention includes measuring the dimensionsand/or shape parameters of the contralateral meniscus. Such dimensionsand parameters include, for example, but are not limited to, the maximumanterior-posterior distance of the meniscus, the maximum medial-lateraldistance of the meniscus, the size or area of the meniscalattachment(s), the maximum length of the anterior horn, the maximumlength of the body, the maximum length of the posterior horn, themaximum and minimum height of the anterior horn, the maximum and minimumheight of the body, the maximum and minimum height of the posteriorhorn, the maximum height and minimum height of the meniscus, the maximumand minimum width of the anterior horn, the maximum and minimum width ofthe body, the maximum and minimum width of the posterior horn, meniscalradii, and angles at various locations.

In one embodiment, the meniscus of the opposite compartment can be usedto create a mirror image of the meniscus on the diseased side. Thesemeasurements can then be used to determine meniscal size and/or shape indesigning treatments for the diseased joint. These treatments caninclude, for example, meniscal repair systems, cartilage repair systems,articular repair systems and arthroplasty systems and they can consistof, for example, biologic materials, tissue scaffolds, plastic, metal ormetal alloys or combinations thereof. Therapies can be custom-made,typically utilizing at least one or more of these measurements.Alternatively, a pre-made, “off-the-shelf” component matching or closelymatching at least one or more of these measurements can be selected.

In yet another embodiment, the 3D geometry of the meniscus on theaffected site can be derived from measurements from neighboringarticular surfaces and structures to recreate the shape and size of thediseased meniscus. Such measurements include, for example, but are notlimited to, tibial bone dimensions, such as maximum anterior-posteriordistance, maximum medial-lateral distance, maximum distance from thetibial spine to the edge, width of the tibial spines, height of thetibial spines, area of tibial plateau occupied by tibial spines, depthof tibial plateau, 2D and 3D shape of tibial plateau; femoral condylebone dimensions, such as maximum anterior-posterior distance, maximumsuperior-inferior distance, maximum medial-lateral distance, maximumdistance from the trochlea to the medial or lateral edge; width anddepth of intercondylar notch, curvature at select regions along thefemoral condyle, 2D and 3D shape.

In yet another aspect, when applied to the knee joint the inventionincludes one or more of the following measurements: (1) tibial bonedimensions, for example, maximum anterior-posterior distance, maximummedial-lateral distance, maximum distance from the tibial spine to theedge, width of the tibial spines, height of the tibial spines, area oftibial plateau occupied by tibial spines, depth of tibial plateau, 2Dand 3D shape of tibial plateau; (2) tibial cartilage dimensions,including thickness and shape; (3) femoral condyle bone dimensions, forexample, maximum anterior-posterior distance, maximum superior-inferiordistance, maximum medial-lateral distance, maximum distance from thetrochlea to the medial or lateral edge; width and depth of intercondylarnotch, curvature at select regions along the femoral condyle, 2D and 3Dshape; and (4) femoral cartilage measurements including thickness andshape. These measurements can then be used to estimate meniscal sizeand/or shape for the treatment of joint diseases. These treatments caninclude, for example, meniscal repair systems, cartilage repair systems,articular repair systems and arthroplasty systems and it can consist of,for example, biologic materials, tissue scaffolds, plastic, metal ormetal alloys, or combinations thereof. Therapies can be custom-made,typically utilizing at least one or more of these measurements.Alternatively, a pre-made, “off-the-shelf” component closely matching atleast one or more of these measurements can be selected.

In a further aspect, meniscal measurements are taken from a referencepopulation possessing normal or near normal menisci. Meniscalmeasurements can include, but are not limited to, for example, themaximum anterior-posterior distance of the meniscus, the maximummedial-lateral distance of the meniscus, the size or area of themeniscal attachment(s), the maximum length of the anterior horn, themaximum length of the body, the maximum length of the posterior horn,the maximum and minimum height of the anterior horn, the maximum andminimum height of the body, the maximum and minimum height of theposterior horn, the maximum height and minimum height of the meniscus,the maximum and minimum width of the anterior horn, the maximum andminimum width of the body, the maximum and minimum width of theposterior horn, meniscal radii and angles at various locations.

Additional non-meniscal measurements can also be taken using the samereference population and may include one or more of the following:

(1) tibial bone dimensions, for example, maximum anterior-posteriordistance, maximum medial-lateral distance, maximum distance from thetibial spine to the edge, width of the tibial spines, height of thetibial spines, area of tibial plateau occupied by tibial spines, depthof tibial plateau, 2D and 3D shape of tibial plateau; (2) tibialcartilage dimensions including thickness and shape; (3) femoral condylebone dimensions, for example, maximum anterior-posterior distance,maximum superior-inferior distance, maximum medial-lateral distance,maximum distance from the trochlea to the medial or lateral edge, widthand depth of the intercondylar notch, curvature at select regions alongthe femoral condyle, 2D and 3D shape, (4) femoral cartilage measurementsincluding thickness and shape; (5) measuring the patellar bonedimensions; (6) measuring the patellar cartilage dimensions includingthickness and shape; and/or (7) measuring the size, length or shape ofligamentous structures such as the cruciate ligaments.

The size and/or shape of the menisci in the reference population canthen be correlated to one or more of the additional non-meniscalmeasurements. Once a correlation is established, the bone and/orcartilage and/or ligamentous dimensions with the highest correlation tomeniscal size and/or shape can be used to predict meniscal size and/orshape in designing therapies for persons suffering from joint disease.The data from the reference population is typically stored in a databasewhich can be periodically or continuously updated. Using thisinformation, therapies can be devices which include, for example,meniscal repair systems, cartilage repair systems, articular repairsystems and arthroplasty systems and they can consist of, for example,biologic materials, tissue scaffolds, plastic, metal or metal alloys, orcombinations thereof. Therapies can be custom-made, typically utilizingat least one or more of these measurements. Alternatively, a pre-made,“off-the-shelf” component closely matching at least one or more of thesemeasurements can be selected. For example, a meniscal repair system canbe selected utilizing this information. Alternatively, this informationcan be utilized in shaping an interpositional arthroplasty system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an example of a Placido disk of concentricallyarranged circles of light. FIG. 1B illustrates an example of a projectedPlacido disk on a surface of fixed curvature.

FIG. 2 shows a reflection resulting from a projection of concentriccircles of light (Placido Disk) on each femoral condyle, demonstratingthe effect of variation in surface contour on the reflected circles.

FIG. 3 illustrates an example of a 2D topographical map of anirregularly curved surface.

FIG. 4 illustrates an example of a 3D topographical map of anirregularly curved surface.

FIG. 5 illustrates surface registration of MRI surface and a digitizedsurface using a laser scanner. The illustration to the left shows thesurface before registration and the illustration to the right shows thesurface after registration.

FIG. 6 is a reproduction of a three-dimensional thickness map of thearticular cartilage of the distal femur. Three-dimensional thicknessmaps can be generated, for example, from ultrasound, CT or MRI data.Dark holes within the substances of the cartilage indicate areas of fullthickness cartilage loss.

FIG. 7 illustrates the cartilage surface of a medial femoral condylefrom a sagittal scan (blue) and a coronal scan (red).

FIG. 8A illustrates an axial view of a meniscus; FIG. 8B illustrates asagittal view of the meniscus; and FIG. 8C illustrates a coronal view ofthe meniscus.

FIG. 9A illustrates a sagittal view of the tibia; and FIG. 9Billustrates a coronal view of the tibia.

FIG. 10A illustrates a sagittal view of the femur; and FIG. 10Billustrates a coronal view of the femur.

FIGS. 11A-C illustrate a chart showing the tibial cartilage surface andsuperior meniscal surface combined after extraction from a coronal FSE,and a meniscal surface scaled to account for compression under loadingconditions. From the information is derived the cross-section of theimplant, FIG. 11C.

FIG. 12 illustrates a point cloud of an implant surface (yellow) thatapproximates smooth surface patch (brown).

FIG. 13A and B are views of an implant suitable for use on a condyle ofthe femur shown from the inferior and superior surface viewpoints,respectively.

FIG. 14 is a view of an implant suitable for a portion of the tibialplateau in the knee.

FIG. 15A-D are views of an implant suitable for the hip.

DETAILED DESCRIPTION OF THE INVENTION

The following description is presented to enable any person skilled inthe art to make and use the invention. Various modifications to theembodiments described will be readily apparent to those skilled in theart, and the generic principles defined herein can be applied to otherembodiments and applications without departing from the spirit and scopeof the present invention as defined by the appended claims. Thus, thepresent invention is not intended to be limited to the embodimentsshown, but is to be accorded the widest scope consistent with theprinciples and features disclosed herein. To the extent necessary toachieve a complete understanding of the invention disclosed, thespecification and drawings of all issued patents, patent publications,and patent applications cited in this application are incorporatedherein by reference.

As will be appreciated by those of skill in the art, the practice of thepresent invention employs, unless otherwise indicated, conventionalmethods of x-ray imaging and processing, x-ray tomosynthesis, ultrasoundincluding A-scan, B-scan and C-scan, computed tomography (CT scan),magnetic resonance imaging (MRI), optical coherence tomography, singlephoton emission tomography (SPECT) and positron emission tomography(PET) within the skill of the art. Such techniques are explained fullyin the literature and need not be described herein. See, e.g., X-RayStructure Determination: A Practical Guide, 2nd Edition, editors Stoutand Jensen, 1989, John Wiley & Sons, publisher; Body CT: A PracticalApproach, editor Slone, 1999, McGraw-Hill publisher; X-ray Diagnosis: APhysician's Approach, editor Lam, 1998 Springer-Verlag, publisher; andDental Radiology: Understanding the X-Ray Image, editor LaetitiaBrocklebank 1997, Oxford University Press publisher.

The present invention solves the need for methods to recreate natural ornear natural relationships between two articular surfaces by providingmethods for determining meniscal size and shape. Meniscal size and shapecan be useful in designing therapies for the treatment of joint diseasesincluding, for example, meniscal repair, meniscal regeneration, andarticular repair therapies.

I. Assessment of Joints

The methods and compositions described herein can be used to treatdefects resulting from disease of the cartilage (e.g., osteoarthritis),bone damage, cartilage damage, trauma, and/or degeneration due tooveruse or age. The invention allows, among other things, a healthpractitioner to evaluate and treat such defects.

As will be appreciated by those of skill in the art, size, curvatureand/or thickness measurements can be obtained using any suitabletechnique. For example, one dimensional, two dimensional, and/or threedimensional measurements can be obtained using suitable mechanicalmeans, laser devices, electromagnetic or optical tracking systems,molds, materials applied to the articular surface that harden and“memorize the surface contour,” and/or one or more imaging techniquesknown in the art. Measurements can be obtained non-invasively and/orintraoperatively (e.g., using a probe or other surgical device). As willbe appreciated by those of skill in the art, the thickness of the repairdevice can vary at any given point depending upon the depth of thedamage to the cartilage and/or bone to be corrected at any particularlocation on an articular surface.

A. Imaging Techniques

As will be appreciated by those of skill in the art, imaging techniquessuitable for measuring thickness and/or curvature (e.g., of cartilageand/or bone) or size of areas of diseased cartilage or cartilage lossinclude the use of x-rays, magnetic resonance imaging (MRI), computedtomography scanning (CT, also known as computerized axial tomography orCAT), optical coherence tomography, SPECT, PET, ultrasound imagingtechniques, and optical imaging techniques. (See, also, InternationalPatent Publication WO 02/22014 to Alexander, et al., published Mar. 21,2002; U.S. Pat. No. 6,373,250 to Tsoref et al., issued Apr. 16, 2002;and Vandeberg et al. (2002) Radiology 222:430-436). Contrast or otherenhancing agents can be employed using any route of administration, e.g.intravenous, intra-articular, etc.

In certain embodiments, CT or MRI is used to assess tissue, bone,cartilage and any defects therein, for example cartilage lesions orareas of diseased cartilage, to obtain information on subchondral boneor cartilage degeneration and to provide morphologic or biochemical orbiomechanical information about the area of damage. Specifically,changes such as fissuring, partial or full thickness cartilage loss, andsignal changes within residual cartilage can be detected using one ormore of these methods. For discussions of the basic NMR principles andtechniques, see MRI Basic Principles and Applications, Second Edition,Mark A. Brown and Richard C. Semelka, Wiley-Liss, Inc. (1999). For adiscussion of MRI including conventional T1 and T2-weighted spin-echoimaging, gradient recalled echo (GRE) imaging, magnetization transfercontrast (MTC) imaging, fast spin-echo (FSE) imaging, contrast enhancedimaging, rapid acquisition relaxation enhancement (RARE) imaging,gradient echo acquisition in the steady state (GRASS), and drivenequilibrium Fourier transform (DEFT) imaging, to obtain information oncartilage, see Alexander, et al., WO 02/22014. Thus, in preferredembodiments, the measurements produced are based on three-dimensionalimages of the joint obtained as described in Alexander, et al., WO02/22014 or sets of two-dimensional images ultimately yielding 3Dinformation. Two-dimensional and three-dimensional images, or maps, ofthe cartilage alone or in combination with a movement pattern of thejoint, e.g. flexion-extension, translation and/or rotation, can beobtained. Three-dimensional images can include information on movementpatterns, contact points, contact zone of two or more opposing articularsurfaces, and movement of the contact point or zone during joint motion.Two and three-dimensional images can include information on biochemicalcomposition of the articular cartilage. In addition, imaging techniquescan be compared over time, for example to provide up-to-date informationon the shape and type of repair material needed.

Any of the imaging devices described herein can also be usedintra-operatively (see, also below), for example using a hand-heldultrasound and/or optical probe to image the articular surfaceintra-operatively.

B. Intraoperative Measurements

Alternatively, or in addition to, non-invasive imaging techniquesdescribed above, measurements of the size of an area of diseasedcartilage or an area of cartilage loss, measurements of cartilagethickness and/or curvature of cartilage or bone can be obtainedintraoperatively during arthroscopy or open arthrotomy. Intraoperativemeasurements may or may not involve actual contact with one or moreareas of the articular surfaces.

Devices suitable for obtaining intraoperative measurements of cartilageor bone or other articular structures, and to generate a topographicalmap of the surface include but are not limited to, Placido disks andlaser interferometers, and/or deformable materials or devices. (See, forexample, U.S. Pat. Nos. 6,382,028 to Wooh et al., issued May 7, 2002;6,057,927 to Levesque et al., issued May 2, 2000; 5,523,843 to Yamane etal. issued Jun. 4, 1996; 5,847,804 to Sarver et al. issued Dec. 8, 1998;and 5,684,562 to Fujieda, issued Nov. 4, 1997).

FIG. 1A illustrates a Placido disk of concentrically arranged circles oflight. The concentric arrays of the Placido disk project well-definedcircles of light of varying radii, generated either with laser or whitelight transported via optical fiber. The Placido disk can be attached tothe end of an endoscopic device (or to any probe, for example ahand-held probe) so that the circles of light are projected onto thecartilage surface. FIG. 1B illustrates an example of a Placido diskprojected onto the surface of a fixed curvature. One or more imagingcameras can be used (e.g., attached to the device) to capture thereflection of the circles. Mathematical analysis is used to determinethe surface curvature. The curvature can then, for example, bevisualized on a monitor as a color-coded, topographical map of thecartilage surface. Additionally, a mathematical model of thetopographical map can be used to determine the ideal surface topographyto replace any cartilage defects in the area analyzed.

FIG. 2 shows a reflection resulting from the projection of concentriccircles of light (Placido disk) on each femoral condyle, demonstratingthe effect of variation in surface contour on reflected circles.

Similarly a laser interferometer can also be attached to the end of anendoscopic device. In addition, a small sensor can be attached to thedevice in order to determine the cartilage surface or bone curvatureusing phase shift interferometry, producing a fringe pattern analysisphase map (wave front) visualization of the cartilage surface. Thecurvature can then be visualized on a monitor as a color coded,topographical map of the cartilage surface. Additionally, a mathematicalmodel of the topographical map can be used to determine the idealsurface topography to replace any cartilage or bone defects in the areaanalyzed. This computed, ideal surface, or surfaces, can then bevisualized on the monitor, and can be used to select the curvature, orcurvatures, of the replacement cartilage.

One skilled in the art will readily recognize that other techniques foroptical measurements of the cartilage surface curvature can be employedwithout departing from the scope of the invention. For example, a2-dimentional or 3-dimensional map, such as that shown in FIG. 3 andFIG. 4 can be generated.

Mechanical devices (e.g., probes) can also be used for intraoperativemeasurements, for example, deformable materials such as gels, molds, anyhardening materials (e.g., materials that remain deformable until theyare heated, cooled, or otherwise manipulated). See, e.g., WO 02/34310 toDickson et al., published May 2, 2002. For example, a deformable gel canbe applied to a femoral condyle. The side of the gel pointing towardsthe condyle can yield a negative impression of the surface contour ofthe condyle. The negative impression can then be used to determine thesize of a defect, the depth of a defect and the curvature of thearticular surface in and adjacent to a defect. This information can beused to select a therapy, e.g. an articular surface repair system. Inanother example, a hardening material can be applied to an articularsurface, e.g. a femoral condyle or a tibial plateau. The hardeningmaterial can remain on the articular surface until hardening hasoccurred. The hardening material can then be removed from the articularsurface. The side of the hardening material pointing towards thearticular surface can yield a negative impression of the articularsurface. The negative impression can then be used to determine the sizeof a defect, the depth of a defect and the curvature of the articularsurface in and adjacent to a defect. This information can then be usedto select a therapy, e.g. an articular surface repair system. In someembodiments, the hardening system can remain in place and form theactual articular surface repair system.

In certain embodiments, the deformable material comprises a plurality ofindividually moveable mechanical elements. When pressed against thesurface of interest, each element can be pushed in the opposingdirection and the extent to which it is pushed (deformed) can correspondto the curvature of the surface of interest. The device can include abrake mechanism so that the elements are maintained in the position thatconforms to the surface of the cartilage and/or bone. The device canthen be removed from the patient and analyzed for curvature.Alternatively, each individual moveable element can include markersindicating the amount and/or degree it is deformed at a given spot. Acamera can be used to intra-operatively image the device and the imagecan be saved and analyzed for curvature information. Suitable markersinclude, but are not limited to, actual linear measurements (metric orempirical), different colors corresponding to different amounts ofdeformation and/or different shades or hues of the same color(s).Displacement of the moveable elements can also be measured usingelectronic means.

Other devices to measure cartilage and subchondral bone intraoperativelyinclude, for example, ultrasound probes. An ultrasound probe, preferablyhandheld, can be applied to the cartilage and the curvature of thecartilage and/or the subchondral bone can be measured. Moreover, thesize of a cartilage defect can be assessed and the thickness of thearticular cartilage can be determined. Such ultrasound measurements canbe obtained in A-mode, B-mode, or C-mode. If A-mode measurements areobtained, an operator can typically repeat the measurements with severaldifferent probe orientations, e.g. mediolateral and anteroposterior, inorder to derive a three-dimensional assessment of size, curvature andthickness.

One skilled in the art will easily recognize that different probedesigns are possible using the optical, laser interferometry, mechanicaland ultrasound probes. The probes are preferably handheld. In certainembodiments, the probes or at least a portion of the probe, typicallythe portion that is in contact with the tissue, can be sterile.Sterility can be achieved with use of sterile covers, for examplesimilar to those disclosed in WO 99/08598A1 to Lang, published Feb. 25,1999.

Analysis on the curvature of the articular cartilage or subchondral boneusing imaging tests and/or intraoperative measurements can be used todetermine the size of an area of diseased cartilage or cartilage loss.For example, the curvature can change abruptly in areas of cartilageloss. Such abrupt or sudden changes in curvature can be used to detectthe boundaries of diseased cartilage or cartilage defects.

II. Segmentation of Articular Cartilage, Bone and Menisci

A semi-automated segmentation approach has been implemented based on thelive wire algorithm, which provides a high degree of flexibility andtherefore holds the potential to improve segmentation of osteoarthriticcartilage considerably. Images are optionally pre-processed using anon-linear diffusion filter. The live wire algorithm assigns a list offeatures to each oriented edge between two pixels (boundary element-bel)in an image. Using an individual cost function for each feature, thefeature values are converted into cost values. The costs for eachfeature are added up by means of a predetermined weighting scheme,resulting in a single joint cost value between 0 and 1 for each bel bthat expresses the likelihood of b being part of the cartilage boundary.To determine the contour of a cartilage object, the operator chooses astarting pixel P. Subsequently, the system calculates the least cost belpath from each image pixel to P with a dynamic programming scheme. Whenthe operator selects another pixel, the system displays the calculatedpath from the current mouse position to P in real time. This currentpath can be frozen as part of the cartilage contour by the operator.This way, the operator has to assemble the desired contour in each slicefrom a number of pieces (“strokes”).

The features of a bel b used with this segmentation technique are thegray values left and right of b and the magnitude of the gray levelgradient across b.

As will be appreciated by those of skill in the art, all or a portion ofthe segmentation processes described can be automated as desired. Aswill be appreciated by those of skill in the art, other segmentationtechniques including but not limited to thresholding, grey levelgradient techniques, snakes, model based segmentation, watershed,clustering, statistical segmentation, filtering including lineardiffusion filtering can be employed.

III. Validation of Cartilage Surface Segmentation

In order to validate the accuracy of the segmentation technique for thearticular cartilage surface, the cartilage surface extracted from MRIscans can be compared with results obtained from segmentation of thejoint surface data which is acquired, for example, using a laser scannerafter specimen dissection. The resulting two surfaces from MRI and laserscan can be registered using the iterative closest point method, and thedistance between each point on the MRI surface to the registered laserscan surface can be used to determine the accuracy of the MRIsegmentation results. FIG. 5 shows the MRI and digitized surfaces beforeand after registration. The distance measurements for the two specimensare shown in TABLE 1.

TABLE 1 DISTANCE CALCULATIONS BETWEEN SEGMENTED MRI AND LASER DIGITIZEDSURFACES (IN MM) Minimum Maximum Mean Standard Specimen DistanceDistance Distance M Deviation σ 1 3.60447e−05 2.10894 0.325663 0.3128032 2.79092e−06 1.616828 0.262131 0.234424

In this example, the data illustrate that the average error between thesegmented MRI surface and the laser scan surface is within the range ofthe resolution of the MRI scan. Thus, the segmentation approach yieldsan accuracy within the given MRI scan parameters.

IV. Calculation and Visualization of Cartilage Thickness Distribution

A suitable approach for calculating the cartilage thickness is based ona 3D Euclidean distance transform (EDT). An algorithm by Saito andToriwaki can be used to achieve computationally very fast (less than 10sec for a 256×256×60 data set on a SGI O2) data processing. Thealgorithm functions by decomposing the calculation into a series of 3one-dimensional transformations and uses the square of the actualdistances. This process accelerates the analysis by avoiding thedetermination of square roots. For initialization, voxels on the innercartilage surface (ICS) are given a value of 0, whereas all othervoxels, including the ones on the outer cartilage surface (OCS) are setto 1.

First, for a binary input picture F={f_(ijk)} (1≦i≦L, 1≦j≦M, 1≦k≦N) anew picture G={g_(ijk)} is derived using equation 1 (α, β, and γ denotethe voxel dimensions).

g _(ijk)=min_(x){(α(i−x))² ;f _(xjk)=0;1≦x≦L}  [Eq. 1]

Thus, each point is assigned the square of the distance to the closestfeature point in the same row in i-direction. Second, G is convertedinto H={h_(ijk)} using equation 2.

h _(ijk)=min_(y) {g _(iyk)(β(j−y))²;1≦y≦M}  [Eq. 2]

The algorithm searches each column in j-direction. According to thePythagorean theorem, the sum of the square distance between a point(i,j,k) and a point (i,y,k) in the same column, (β(j−y))², and thesquare distance between (i,y,k) and a particular feature point, g_(iyk),equals the square distance between the point (i,j,k) and that featurepoint. The minimum of these sums is the square distance between (i,j,k)and the closest feature point in the two-dimensional i-j-plane.

The third dimension is added by equation 3, which is the sametransformation as described in equation 2 for the k-direction.

s _(ijk)=min_(z) {h _(ijz)+(γ(k−z))²;1≦z≦N}  [Eq. 3]

After completion of the EDT, the thickness of the cartilage for a givenpoint (a,b,c) on the OCS equals the square root of s_(abc). This resultsin a truly three-dimensional distance value determined normal to theICS. The x, y, and z position of each pixel located along thebone-cartilage interface is registered on a 3D map and thickness valuesare translated into color values. In this fashion, the anatomic locationof each pixel at the bone-cartilage interface can be displayedsimultaneously with the thickness of the cartilage for that givenlocation (FIG. 6).

As will be appreciated by those of skill in the art, other techniquesfor calculating cartilage thickness can be applied, for example usingthe LaPlace equation, without departing from the scope of the invention.

V. Calculation and Visualization of Cartilage Curvature Distribution

Another relevant parameter for the analysis of articular cartilagesurfaces is curvature. In a fashion similar to the thickness map, a setof curvature maps can be derived from the cartilage surface data that isextracted from the MRI.

A local bi-cubic surface patch is fitted to the cartilage surface basedon a sub-sampling scheme in which every other surface point is used togenerate a mesh of 5×5 point elements. Thus, before performing the fitthe density of the data is reduced in order to smooth the fitted surfaceand to reduce the computational complexity.

After computation of the local bi-cubic surface fits, the unit normalvectors {n} are implicitly estimated from the surface fit data. Thecorresponding curvature and its orientation are then given by:

K _(i)=arccos(n ₀ ·n _(i))/ds _(i) =dθ/ds _(i),

where n_(o) is the unit normal vector at the point (u, v) where thecurvature is being estimated and n, (i=1, . . . , 24) are the unitnormal vectors at each one of the surrounding points in the 5×5 localsurface patch. FIG. 6 shows an example of the maximum principalcurvature maps (value and direction), estimated using the bi-cubicsurface patch fitting approach.

As will be appreciated by those of skill in the art, other techniques,such as n-degree polynomial surface interpolation or approximation,parametric surface interpolation or approximation and different discretecurvature estimation methods for measuring curvature or 3D shape can beapplied.

VI. Fusion of Image Data from Multiple Planes

Recently, technology enabling the acquisition of isotropic ornear-isotropic 3-dimensional image data has been developed. However,most MRI scans are still acquired with a slice thickness that is 3 ormore times greater than the in-plane resolution. This leads tolimitations with respect to 3D image analysis and visualization. Thestructure of 3-dimensional objects cannot be described with the sameaccuracy in all three dimensions. Partial volume effects hinderinterpretation and measurements in the z-dimension to a greater extentthan in the x-y plane.

To address the problems associated with non-isotropic image resolutions,one or more first scans S1 are taken in a first plane. Each of the firstscans are parallel to each other. Thereafter, one or more second scansS2 are taken with an imaging plane oriented to the first scan S1 so thatthe planes intersect. For example, scans S1 can be in a first planewhile scans S2 are in a plane perpendicular to the first plane.Additional scans in other planes or directions, e.g., S3, S4. Sn, canalso be obtained in addition to the perpendicular scans or instead ofthe perpendicular scans. S2, and any other scans, can have the samein-plane resolution as S1. Any or all of the scans can also contain asufficient number of slices to cover the entire field of view of S1. Inthis scenario, two data volumes with information from the same 3D spaceor overlapping 3D spaces can be generated.

Data can be merged from these two scans to extract the objects ofinterest in each scan independently. Further, a subsequent analysis cancombine these two segmented data sets in one coordinate system, as isshown in FIG. 6. This technique is helpful in outlining the boundariesof objects that are oriented parallel to the imaging plane of S1, buttherefore will be perpendicular to the imaging plane of S2.

For quantitative measurements, such as determining the cartilage volume,it can be advantageous to combine S1 and S2 directly into a third datavolume. This third data volume is typically isotropic or near-isotropicwith a resolution corresponding to the in-plane resolution of S1 and S2,thus reducing partial volume effects between slices (FIG. 7). S1 and S2can first be registered into the same coordinate system. If both scansare acquired during the same session (without moving the patient betweenscans), the image header information is used to obtain thetransformation matrix. Otherwise, a mutual information-based rigidregistration is applied. The gray value for each voxel V of the thirddata volume is calculated as follows:

(1) determine the position in 3D space for V;

(2) determine the gray values in S1 and S2 at this position;

(3) interpolate the two gray values into a single gray value G; and

(4) assign G to V.

As an alternative to fusion of two or more imaging planes, data can beobtained with isotropic or near isotropic resolution. This is possible,for example, with spiral CT acquisition technique or novel MRI pulsesequence such as 3D acquisition techniques. Such 3D acquisitiontechniques include 3D Driven Equilibrium Transfer (DEFT), 3D FastSpin-Echo (FSE), 3D SSFP (Steady State Free Precession), 3D GradientEcho (GRE), 3D Spoiled Gradient Echo (SPGR), and 3D Flexible EquilibriumMR (FEMR) techniques. Images can be obtained using fat saturation orusing water selective excitation. Typically, an isotropic resolution of0.5×0.5×0.5 mm or less is desirable, although in select circumstances1.0×1.0×1.0 and even larger can yield adequate results. With nearisotropic resolution, the variation in voxel dimensions in one or moreplanes does not usually exceed 50%.

VII. In Vivo Measurement of Meniscal Dimensions

The dimensions and shape of a personalized interpositional arthroplastysystem can be determined by measuring a patient's meniscal shape andsize and by evaluating the 3D geometry of the articular cartilage. Manyosteoarthritis patients, however, have torn menisci, often times withonly small or no meniscal remnants. In these patients, the shape of apersonalized interpositional arthroplasty system can be determined byacquiring measurements of surrounding articular surfaces and structures.

In the knee, for example, a few measurements can be made on the femoraland tibial bone in MR images of the diseased knee. For optimal fit, theshape of the superior surface of the implant should resemble that of thesuperior surface of the respective meniscus. Measurements of the bonescan help determine how well meniscal dimensions can be predicted.

FIG. 8A illustrates an axial view of a meniscus 100. The meniscus has amaximum anterior-posterior distance 1, and a maximum medial lateraldistance 2. In the knee, the meniscus compensates for an anterior hornand a posterior which each have a maximum length 3, 5 and width 9, 11.The body itself has a maximum length 4 and width 10 which are a functionof the patient's anatomy. FIG. 8B illustrates a sagittal view of themeniscus in FIG. 8A. The meniscus 100 has a maximum height 6, 8 whichcorrelates to the maximum height of the anterior horn and the posteriorhorn. FIG. 8C illustrates a coronal view of the meniscus 100. From thecoronal view it is apparent that the body has a maximum and minimumheight.

Turning now to FIG. 9A, a sagittal view of a tibia 110 is shown. Thetibia has a maximum anterior-posterior distance 12. FIG. 9B illustratesthe coronal view of the tibia 110 shown in FIG. 9A. From the sagittalview it is apparent that the tibia has a maximum medial-lateral distance13, a maximum distance from the tibial spine to the edge 14, and a width15.

The tibia mates with the femur 120, which is shown in a sagittal view inFIG. 10A. The femur has a maximum anterior-posterior distance 16 and amaximum superior-interior distance 17. From the coronal view shown inFIG. 10B the maximum medial-lateral distance 18, the distance from thetrochlea to the edge 19, and the width of the intercondylar notch 20 isapparent.

A Pearson's correlation coefficient r can be obtained for a variety ofmeasurements to assess how well one variable is expressed by anothervariable. Suitable measurements include, for example, the followingmeasurements:

-   -   antero-posterior (AP) length of medial (lateral) meniscus with        AP length of medial (lateral) femoral condyle;    -   AP length of medial (lateral) meniscus with AP length of medial        (lateral) tibial plateau;    -   medio-lateral (ML) width of medial (lateral) meniscus with ML        width of medial (lateral) femoral condyle;    -   ML width of medial (lateral) meniscus with ML width of medial        (lateral) tibial plateau;    -   Y coordinate of highest point of medial (lateral) meniscus with        y coordinate of highest point of medial (lateral) tibial spine;    -   X coordinate of medial (lateral) margin of medial (lateral)        meniscus with x coordinate of medial (lateral) margin of medial        (lateral) femoral condyle; and    -   X coordinate of medial (lateral) margin of medial (lateral)        meniscus with x coordinate of medial (lateral) margin of medial        (lateral) tibial plateau.

Examples of measurements obtained are summarized in TABLE 2.

TABLE 2 CORRELATION BETWEEN MENISCAL DIMENSIONS AND DIMENSIONS OFFEMORAL AND TIBIAL BONE Measurement Imaging Plane N Pearson's r APLength: medial meniscus-medial Sagittal 23 0.74 femoral condyle APLength: lateral meniscus-lateral Sagittal 24 0.73 femoral condyle APLength: medial meniscus-medial Sagittal 23 0.79 tibial plateau APLength: lateral meniscus-lateral Sagittal 24 0.27 tibial plateau MLWidth: menisci-femur Coronal 12 0.91 ML Width: menisci-tibia Coronal 120.92 ML Width: menisci-medial femoral Coronal 12 0.81 condyle ML Width:menisci-lateral femoral Coronal 12 0.65 condyle ML Width: menisci-medialtibial Coronal 12 0.86 plateau ML Width: menisci-lateral tibial Coronal12 0.48 plateau ML Width: medial meniscus-medial Coronal 12 0.95 femoralcondyle ML Width: lateral meniscus-lateral Coronal 12 0.45 femoralcondyle ML Width: medial meniscus-medial Coronal 12 0.69 tibial plateauML Width: lateral meniscus-lateral Coronal 12 0.34 tibial plateau MLLength: medial meniscus-lateral Coronal 12 0.12 meniscus MeniscalHeight: medial meniscus- Coronal 12 0.01 lateral meniscus MeniscalHeight: Medial meniscal Coronal 12 0.22 height-medial femoral heightMeniscal Height: Lateral meniscal Coronal 12 0.22 height-lateral femoralheight Meniscal Height: Medial meniscal Coronal 12 0.55 height-medialtibial height Meniscal Height: Lateral meniscal Coronal 12 0.17height-lateral tibial height Highest Point (y coordinate): medialCoronal 12 0.99 meniscus-medial tibial spine Highest Point (ycoordinate): lateral Coronal 12 0.90 meniscus-lateral tibial spineMedial margin (x-coordinate): medial Coronal 12 1.00 meniscus-femoralcondyle Lateral margin (x-coordinate): lateral Coronal 12 1.00meniscus-lateral femoral condyle Medial Margin (x-coordinate): medialCoronal 12 1.00 meniscus-medial tibial plateau Lateral Margin(x-coordinate): lateral Coronal 12 1.00 meniscus-lateral tibial plateau

The Pearsons' coefficient determines the relationship between two sizesthat are measured. The higher the correlation, the better therelationship between two measurements. From the data in TABLE 2, itbecomes evident that, in the knee, the AP length of both medial andlateral menisci can be predicted well by measuring the length of therespective femoral condyle. For the medial meniscus, the length of themedial tibial plateau can also be used. The ML width of the medialfemoral condyle is a good predictor for the width of the medialmeniscus. The height of the medial and lateral tibial spines correlateswell with the height of the respective menisci. Correlations between MLwidth of the lateral meniscus and width of the lateral femoral condyleand tibial spine are lower due to a high variability of the most lateralpoint of the lateral meniscus. As opposed to these outermost points ofthe lateral meniscus, the main margins correlate very well with themargins of the tibia and femur. This is also the case for the medialmeniscus. Consequently, the outer margins of medial and lateral meniscican be determined.

These results show that meniscal dimensions can be predicted in areliable fashion by measuring bony landmarks in MR images. Where thePearson's coefficient is high (e.g., close to 1), the two measurementscan, in effect, be used interchangeably to represent the measurementdesired. Where the Pearson's coefficient is low (e.g., 0.34), acorrection factor may be applied to the measurement. The measurement ascorrected may then equal or approximate the corresponding measurement.In some instances, use of a correction factor may not be feasible ordesired. In that instance, other approaches, such as logistic regressionand multivariate analysis, can be used as an alternative withoutdeparting from the scope of the invention.

A person of skill in the art will appreciate that while this data hasbeen presented with respect to the meniscus in the knee and measurementof knee anatomy relative thereto, similar results would occur in otherjoints within a body as well. Further, it is anticipated that a libraryof measurements can be created, for example for generating one or morecorrelation factors that can be used for a particular joint. Forexample, a single correlation factor can be generated using a pluralityof measurements on different subjects.

Alternatively, a plurality of correlation factors can be generated basedon, for example, joint assessed, size, weight, body mass index, age, sexof a patient, ethnic background. In this scenario, a patient seekingtreatment can be assessed. Measurements can be taken of, for example,the medial femoral condyle. The correlation factor for the medialfemoral condyle in the patient can then be compared to a correlationfactor calculated based on samples wherein the sample patients had thesame, or were within a defined range for factors, including for example:size, weight, age and sex.

VIII. Surface Digitization

Digitized surface data from menisci of cadaveric specimens forgeneration of a generic meniscal model can be acquired using a TitaniumFaroArm® coordinate measurement machine (CMM) (FARO Technologies Inc.,Lake Mary, Fla.).

IX. 3D Design Techniques for Anatomically Correct InterpositionalArthroplasty System

The design workflow for each implant can consist of a combination of oneor more of the following steps:

-   -   a. Fusion of the sagittal and coronal 3D SPGR or 2D or 3D FSE        data or other sequences for a joint;    -   b. Segmentation of point data from the cartilage surface of a        joint;    -   c. Fusion of the sagittal and coronal 2D or 3D FSE or 2D SE data        or other sequences of a joint;    -   d. Segmentation point data of the superior meniscal surface;    -   e. Combination of cartilage surface data and meniscal surface        data to serve as model for a surface of an implant;    -   f. Compression of a meniscal surface by factor ranging from 0.2        to 0.99;    -   g. Conversion of point cloud data for a superior and an inferior        implant surface into parametric surface data; and    -   h. Cutting of parametric surface data sets to determine exact        shape of implant.

In many patients with advanced osteoarthritis, however, the meniscus is,to a great extent, depleted, and therefore cannot serve directly as atemplate from which the superior implant surface can be derived. Inthese cases, dimensions of the remaining joint bone, can be used toadjust the size of a generic meniscal model, which can then serve as atemplate for the implant.

X. Derivation of Implant Surfaces from Cartilage and Healthy MeniscalSurfaces

The superior surface of an implant can be modeled based on the superiormeniscal surface and the joint cartilage surface in those areas that arenot covered by the meniscus. Therefore, after the slice-by-slicesegmentation of the superior meniscal surface from the SE or FSE orother MRI images and the tibial cartilage surface from the 3D SPGR orFSE or other MRI images, both data sets will be combined (FIGS. 11A-C).To determine the composite surface for the prosthesis, the intersectionbetween the two surfaces is located. In the event that the two surfacesdo not intersect in a particular slice, the intersection between thetangential line through the

-   -   d. Segmentation point data of the superior meniscal surface;    -   e. Combination of cartilage surface data and meniscal surface        data to serve as model for a surface of an implant;    -   f. Compression of a meniscal surface by factor ranging from 0.2        to 0.99;    -   g. Conversion of point cloud data for a superior and an inferior        implant surface into parametric surface data; and    -   h. Cutting of parametric surface data sets to determine exact        shape of implant.

In many patients with advanced osteoarthritis, however, the meniscus is,to a great extent, depleted, and therefore cannot serve directly as atemplate from which the superior implant surface can be derived. Inthese cases, dimensions of the remaining joint bone, can be used toadjust the size of a generic meniscal model, which can then serve as atemplate for the implant.

X. Derivation of Implant Surfaces from Cartilage and Healthy MeniscalSurfaces

The superior surface of an implant can be modeled based on the superiormeniscal surface and the joint cartilage surface in those areas that arenot covered by the meniscus. Therefore, after the slice-by-slicesegmentation of the superior meniscal surface from the SE or FSE orother MRI images and the tibial cartilage surface from the 3D SPGR orFSE or other MRI images, both data sets will be combined (FIGS. 11A-C).To determine the composite surface for the prosthesis, the intersectionbetween the two surfaces is located. In the event that the two surfacesdo not intersect in a particular slice, the intersection between thetangential line through the central end of the meniscal surface with thetibial surface will be calculated (FIG. 11A). To account for naturalcompression of the elastic meniscus under load, its height can beadjusted, for example, to 60% of the original height (FIG. 11B). Forthis purpose, each point on the meniscal surface is connected to theclosest point on the cartilage surface. The new point for the adjustedmeniscal surface is chosen at 60% of the distance from the tibialcartilage surface.

As will be appreciated by those of skill in the art, a variety of otheradjustment ratios can be used without departing from the scope of theinvention. Suitable adjustment ratios will vary depending on patientphysiology and desired degree of correction and include, for example,ratios that range from 0.2 to 1.5. The amount of height adjustment ofthe implant relative to the natural meniscus will vary depending uponthe material that the implant is manufactured from. For example, wherethe implant is manufactured from a material having a high degree ofelasticity, it may be desirable to use an adjustment greater than 1.Where the material has a low degree of elasticity, the adjustment islikely to approach 50%. The appropriate adjustment will also depend uponthe joint for which the implant is manufactured. Thus, for example, animplant manufactured for the knee using a material with a low degree ofelasticity can have an adjustment of between 50-70%, while an implantmanufactured for the shoulder also using a material with a low degree ofelasticity may have a desired adjustment of 60-80%. Persons of skill inthe art will appreciate that the correction factor for an implant willvary depending upon the target joint and the properties of the materialfrom which the implant is manufactured.

The adjustment ratio can also vary depending on the location within ajoint with a plurality of ratios possible for any given design. Forexample, in a knee joint, an adjustment ratio close to 0.8 can be usedanteriorly, while an adjustment ratio close to 0.5 can be usedposteriorly. Additionally, more adjustment ratios can be selected suchthat the adjustment ratio gradually changes, for example, anteriorly,depending on the anticipated biomechanics of the joint. Changes can alsobe made to the adjustment ratio as a result of patient specificparameters such as age, sex, weight, ethnicity, and activity level. Theadjustment ratio can be selected in order to achieve an optimalbiomechanical or functional result. In vitro cadaveric testing,constraint testing, testing of contact surface, fatigue testing androbotic testing can, for example, be used for determining the optimaladjustment ratio(s) for an implant.

Finally, to determine the shape of the superior surface of the implant,the compressed meniscal surface can be combined with the portion of thetibial cartilage surface that is not covered by the meniscus. The shapeof, for example, an inferior surface of the implant can be derived fromthe entire cartilage surface (FIG. 11C) or the subchondral bone surface.The latter can be used, for example, if there is significant eburnationof the joint and most of the cartilage has been lost.

XI. Derivation of Superior Implant Surface in Case of Damaged Meniscus

In patients with a damaged or degenerated meniscus or those that had aprior meniscectomy, the meniscal surface cannot be used as a templatefor an implant surface as described above. In these cases, a genericmeniscal model can be used to design the desired implant surface.

The generic meniscal model can be generated from data that is, forexample, collected from cadaveric femoral specimens using a TitaniumFaroArm as described above. Alternatively, a laser scanning device or anoptical device can be used. In this instance, meniscal surface data canbe digitized, for example, from ten frozen cadaveric tibial specimens.All surface data sets obtained can then be matched for size differencesusing, for example, an affine surface registration scheme. The matchedsurface points after registration can then be merged into a single pointcloud. A generic meniscal surface, S_(g), can be fitted through a pointcloud using a least-squares optimization, resulting in a “mean” surfaceof the ten specimens.

Typically, dimensions of healthy menisci correlate well with dimensionsof bony landmarks. Therefore, measurements of bony landmarks in an MRIcan be used to reconstruct the dimensions of the healthy meniscus (see,e.g., TABLE 2, above). The antero-posterior length L will be calculatedfrom the length of the femoral condyle. For determining medio-lateralmeniscal width W, we can use the position of the medial margin of thetibia for the medial meniscus and the lateral tibial margin for thelateral meniscus. The height H can be derived from the highest point ofthe tibial spine.

Once the values L, W, and H have been determined, S_(g) can be deformedaccordingly. Each point P in S_(g) with the coordinates (x, y, z) can betransformed into a new point P′ using Equation 4:

P′=(x′,y′,z′)=((L/L _(g))·x,(W/W _(g))·y,(H/H _(g))·z)  [Eq. 4]

where L_(g), W_(g), and H_(g) are the respective dimensions of S_(g).The transformed points P′ can form the meniscal surface S that will beused as a template for designing the superior implant surface asdescribed in the previous section.

XII. Final Steps of Implant Design

The first and second implant surfaces derived from an MR image, asdescribed above, consist of point clouds. The point clouds can beconverted into a data format that then can be manipulated in, forexample, a CAD system. The Surface Patch function in the surfacemodeling program Rhinoceros can be used to approximate a smooth surfacepatch to the point cloud data (FIG. 12). This surface can then beexported in the IGES format to be read by the CAD software. Othersoftware programs can be used without departing from the scope of theinvention. For example, Pro/Engineer, Solid Edge, Alibre and IronCAD arealso suitable programs.

Using the CAD software SolidWorks, the superior and inferior surfacescan be combined into one design model. Both surfaces can be clippedusing the outer meniscal edge as a margin (FIG. 11).

From this information, joint implants can be designed that take intoconsideration the dimensions. FIGS. 13A and B are views of a jointimplant suitable for use on a condyle of the femur. These views areshown from the inferior and superior surface viewpoints. The surfaces,edges and height of the implant can be adjusted to account for themeasurements taken to achieve an implant with an optimal patient fit.FIG. 14 is a view of an implant suitable for placement in a joint kneeand placed on a portion of the tibial plateau. FIGS. 15A-D are views ofan implant suitable for the hip. These implants can also be designed sothat the surfaces, edges and height of the implants can be adjusted toaccount for the measurements taken as well as the patient specificcriteria, as appropriate or desirable.

XIII. Accuracy of 3D Imaging and 3D Sizing Techniques for Deriving 3DShape of Implant

In order to determine how much the predicted meniscal surface,calculated from the generic model, differs from the true shape of themeniscus, healthy volunteers can be examined. Suitable spiral CT, alsowith intravenous or intra-articular contrast enhancement, or MRI imagescan be acquired, from which medial and lateral menisci can then beextracted using live wire segmentation, or other suitable mechanisms.Furthermore, the generic models for the medial and lateral meniscus canbe fitted as described above. For each subject, the medial and lateralmeniscus that was segmented from the MRI can be compared to the fittedmodels as follows:

-   -   1. For each point P=(x,y,z) in the segmented data set choose the        closest point P₁=(x₁,y₁,z₁) from the fitted model with z₁≧z and        the two closest points P₂=(x₂,y₂,z₂) and P₃=(x₃,y₃,z₃) with        Z₂,Z₃≦Z.    -   2. The point P is projected orthogonally onto the plane defined        by P₁, P₂ and P₃. The projected point P′ is given by:

P□=P−((P−P ₁ ,n)/(n,n))

-   -   where n is the normal to the plane and (•,•) denotes the dot        product.    -   3. Calculate the distance d₁ between P and the plane, given by

d ₁ =∥P′−P∥.

-   -   4. Repeat 1-3 with P₁=(x₁,y₁,z₁) such that z₁≦z and        P₂=(x₂,y₂,z₂) and P₃=(x₃,y₃,z₃) such that z₂,z₃≧z, resulting in        d₂.    -   5. Calculate the mean distance for P: d(P)=(d₁+d₂)/2.    -   6. Calculate the total distance measure D over all points in the        segmented data set:

D=Σ _(p) d(P).

The total distance measure D depends on the relative position of thesegmented MRI data and the fitted model in the coordinate system. Thisrelative position can be optimized to minimize D by adjusting the rigidbody transformation T that positions the model in an iterativeregistration process based on the iterative closest point algorithm,using D(7) as a cost function.

Typically, it is anticipated that the accuracy of this fitting approachis sufficient if the average distance D/n, where n is the number ofpoints in the segmented data, is below 1.5 mm.

The foregoing description of embodiments of the present invention hasbeen provided for the purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseforms disclosed. Many modifications and variations will be apparent tothe practitioner skilled in the art. The embodiments were chosen anddescribed in order to best explain the principles of the invention andits practical application, thereby enabling others skilled in the art tounderstand the invention and the various embodiments and with variousmodifications that are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the followingclaims and equivalents.

1. A method for designing a device for repairing a joint of a patientcomprising: acquiring at least one of an isotropic image data set ornear isotropic image data set for the joint; segmenting data of at leastone articular surface of the joint; and converting the segmented imagedata into a patient-specific portion of the device, wherein thepatient-specific portion includes at least one patient-specificdimension or shape parameter.